The present invention pertains to fiber optic sensors and particularly to current sensors. More particularly, the invention pertains to current difference measuring.
It is often desirable to measure the difference between current flowing at two points along the same conductor. Such measurements are used for differential fault current relaying in electric power systems. In addition to detecting large faults, the difference current can also give information about the health and remaining life of the insulation surrounding the conductor. For example, small leakage currents to ground through the insulation cause the current at the two points to become unequal.
A difficulty arises when trying to measure small leakage currents to monitor the health and remaining life of the insulation. The difficulty is that important leakage currents may be a factor of one million lower than the main current flowing through the conductor. Thus, to just make a measurement at two points along the conductor and compare the results would require that the two current sensors be calibrated to within one part-per-million of each other. This requirement is excessively difficult to achieve.
A second issue is that the leakage current may be a higher harmonic of the main current flowing in the conductor. The leakage current may only exist when the voltage on the conductor exceeds some threshold; the insulator material may act as a non-linear impedance to ground. To accurately measure the harmonic current level in the two sensors in the presence of the main current requires that both current sensors exhibit extraordinary linear responses. For some applications, the harmonic distortion of the sensors needs to be less than one part-per-million. Again, this requirement is excessively difficult to meet.
Some of the elements of the present current sensor arrangement may utilize sensor technology from the art. Optical fiber current sensor technology is disclosed in U.S. Pat. No. 5,644,397 issued Jul. 1, 1997, to inventor James N. Blake, and entitled xe2x80x9cFiber Optic Interferometric Circuit and Magnetic Field Sensorxe2x80x9d, which is incorporated herein by reference. Similar technology is also disclosed in U.S. Pat. No. 5,696,858 issued Dec. 9, 1997, to inventor James N. Blake, and entitled, xe2x80x9cFiber Optics Apparatus and Method for Accurate Current Sensingxe2x80x9d, which is incorporated herein by reference.
The present invention utilizes first and second optical current sensors placed at two different points in an electrical system. In addition to the main conductor passing through the two current sensors, a common secondary nulling current is provided to pass through each current sensor as well. This nulling current is advantageously generated using a closed loop signal processor connected to the first current sensor. The nulling current then flows through the second current sensor, either by a direct wire connection, or by a wireless (e.g., RF or optical) or wire (e.g., telephone line) communications link which provides the necessary information for a secondary current generator to generate substantially the same nulling current as flows through the first current sensor. The nulling current causes the first sensor to register zero output and is therefore equal and opposite to the main current flowing through the first sensor. As this nulling current passes through the second sensor, the output of the second sensor is the difference between the main current flowing therethrough and the nulling current, or equivalently, the difference between the main currents flowing through the two sensors. Thus the output of the second sensor is substantially equal to the leakage current.